Known rotary atomizers, such as those used for electrostatically coating automobile bodies, are contaminated while in operation by the coating material which they spray. The parts particularly affected are the bell plate casing, the drive shaft area between the spray bell and the front housing part containing the mounting shaft and the external surfaces of the housing part itself. Not only does this unwanted deposit of coating material initially fail to reach the object to be coated, but some of it may also later break away while the unit is in operation and then reach the object, resulting in considerable contamination thereof, especially if there has been a color change in the meanwhile. Therefore, known atomizers must be cleaned frequently and regularly. This is a relatively tedious and time-consuming operation involving lengthy shut-downs which are particularly detrimental to mass production coating.
This self-contamination occurs not only in conventional rotary atomizers (cf. EP Patent No. 0032391), in which the electrostatic field which charges the paint particles is produced between the high voltage spray bell and the grounded body, but to a still greater degree, in apparatuses including a grounded spray head and external electrodes for producing the electrical field, in which there is no potential gradient between the spray head and the object to be coated to oppose the danger of contamination (cf. German OS No. 34 29 075). Apparatuses including external electrodes have the advantage of being suitable for paint materials which do not pollute the environment, contain few organic constituents and are relatively highly conductive. The self-contamination problem, however, in these apparatuses has hitherto substantially prevented the use of paint materials which are not harmful to the environment.